Abstract
Annealing at ambient temperature connected with a substantial degree of strengthening (natural ageing) is well known and investigated in Al-based alloys. Clustering of solute atoms at room temperature proven by atom probe tomography is believed to be assisted by vacancies quenched in material from high annealing temperature.
This process has often a very deteriorating effect on further precipitation during artificial ageing of Al alloys. Natural ageing is not common in Mg-based alloys contrary to Al hardenable alloys.
Two squeeze cast Mg-Gd alloys with different Gd content were solution heat treated. The increase of solute concentration in the matrix during the treatment was monitored by electrical resistivity measurements.
Microhardness decrease during the solution heat treatment corresponds to dissolution of secondary phases combined with solid solution hardening. Electrical resistivity of both alloys measured at 77 K decreases, if the alloys are kept at room temperature immediately after quenching from the solution temperature.
These changes are accompanied by a microhardness increase. The process is much slower compared to Al alloys and does not completely saturate even after 2 months.
Most probably a solute atoms clustering proceeds during the natural ageing as no secondary phase(s) particles were revealed by transmission electron microscopy in the naturally aged alloys. The influence of natural ageing on phase changes due to isochronal annealing from room temperature to 500oC and to isothermal annealing at 200oC was studied by electrical resistometry performed at 77 K and microhardness development measured at room temperature.
Natural ageing in the studied Mg-Gd alloys does not diminish microhardness increase achieved during artificial heat treatment contrary to Al alloys.